<p>For earthquake resistant design, structural degradation is considered using traditional strength modification factors, which are obtained via the ratio of the nonlinear seismic response of degrading and non-degrading structural single degree of freedom (SDOF) systems. In this paper, with the aim to avoid the nonlinear seismic response to compute strength modification factors, a methodology based on probabilistic seismic hazard analyses (PSHA) is proposed in order to obtain strength modification factors of design spectra which consider structural degradation through the spectral-shape intensity measure <i>I<sub>Np</sub></i>. PSHA using <i>I<sub>Np</sub></i> to account for structural degradation, and <i>Sa</i>(<i>T</i><sub>1</sub>) which represents the spectral acceleration associated with the fundamental period and does not consider such degradation, are performed. The ratio of the uniform hazard spectra in terms of <i>I<sub>Np</sub></i> and <i>Sa</i>(<i>T</i><sub>1</sub>), that represent the response of degrading and non-degrading systems, provide new strength modification factors without the need to develop nonlinear time history analysis. A mathematical expression is fitted to the ratios that correspond to systems located in different soil types. The expression is validated by comparing the results with those derived from nonlinear time-history analyses of structural systems.</p>